This invention is concerned with pretreatment, e.g. prefilter structures which have a particular utility in the separation of oil from water in an oil/water system wherein oil is the dispersed phase and water is the continuous phase. The structure of the present invention is particularly useful when the oil/water emulsion is stabilised by the presence of a detergent.
It should be appreciated that as used herein the term "oil" means any organic liquid which is substantially immiscible with water and which either has a specific gravity appreciably from that of water or which exhibits such difference when the specific gravity of the water is altered by a solute dissolved therein.
One means for the separation of oil from oil/water emulsions involves the use of a coalescing filter designed for "inside to outside flow". Such a structure also serves to remove solid impurities in the emulsion. In this technique the emulsion is passed through the wall of a filter tube, e.g. of glass fibres held together by a resinous binder. It is possible to assist in separation efficiency by positioning a coalescing sleeve over the exterior of the tube, e.g. a sleeve of a plastics foam of relatively high pore size (e.g. 80 to 100 p.p.i.). As the emulsion passes through the wall of the filter under pressure oil droplets are coalesced and an oil phase separates from the water passing through the wall.
Unfortunately, whilst a conventional coalescing filter functions satisfactorily in the absence of detergents, when a material having appreciable surfactant properties is present in the oil/water emulsion the oil coalescing efficiency decreases appreciably.